Terminal differentiation of ventricular myocytes involves the coordinate expression of lineage-restricted genes and permanent growth arrest. This irreversible expression of tissue-specific genes and cell cycle exit precludes myocardial regeneration as therapeutic maneuver after infarction of other insult. An understanding of the molecular events leading to cardiac differentiation would provide a strong scientific foundation towards the future objective of manipulation of these factors to regenerate human myocardium. Rb, the protypical member of a family of pocket proteins, has been implicated in directing critical aspects of this process through its association with tissue-specific transcription factors and inactivation of positive cell cycle regulators. One novel and fruitful approach to identify and characterize critical factors mediating terminal differentiation in other tissues has been to exploit E1A's ability to selectively bind and displace endogenous factors from the pocket resulting in downregulation of tissue-restricted genes while forcing cell cycle reetry. The preceptor's lab has recently demonstrated evidence for a pocket protein-dependent pathway that governs cardiac-specific transcription and cell cycle regulation in cardiac muscle. The present application proposes to exploit the pocket-binding properties of adenoviral E1A proteins and two models of Rb deficiency to study the role of pocket proteins in terminal cardiac differentiation. Based on results from skeletal muscle several mechanisms can be envisioned to explain the pocket protein-dependent pathway repression of cardiac genes. They from the basis for the specific aims of this proposal: 1) Does E1A alter activity of cardiogenic factors by directly binding and inactivating cardiogenic factors, decreasing expression or DNA binding, repressing transcriptional activity, 2) Do cardiac transcription factors, analogous to MyoD, require association with Rb or other pocket proteins for transcriptional activity; 3) Are the effects of E1A are secondary to release of E2F from the Rb; 4) Does E1A increase cyclin/Cdk activity in cardiac myocytes; and 5) Investigate in vivo, the role of Rb in controlling terminal differentiation by cardiac-restricted homologous recombination. (End of abstract)